Demographic drivers of Norway rat populations from urban slums in Brazil.

The Norway rat is a globally distributed pest, known for its resilience to eradication and control programs. Efficient population control, especially in urban settings, is dependent on knowledge of rat demography and population ecology. We analyzed the relationship between four demographic outcomes, estimated by live-trapping data, and fine-scale environmental features measured at the capture site. Wounds, a proxy for agonistic interactions, were associated with mature individuals. Areas with environmental features favorable to rats, such as open sewers and unpaved earth, were associated with more mature individuals with a better body condition index. The control measures (environmental stressors) are likely to be disrupting the social structure of rat colonies, increasing the frequency and distribution of agonistic interactions, which were common in both sexes and maturity states. The relationship between the favorable environmental conditions and the demographic markers analyzed indicate possible targets for infestation control through environmental manipulation, and could be incorporated into current pest management programs to achieve long-term success. Our study indicate that urban interventions focused on removal of potential resources for rats could be potential long-term solutions by reducing the carrying capacity of the environment. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11252-020-01075-2.

Prevalence of Diarrheagenic Escherichia coli (DEC) and Salmonella spp. with zoonotic potential in urban rats in Salvador, Brazil.

Studies evaluating the occurrence of enteropathogenic bacteria in urban rats (Rattus spp.) are scarce worldwide, specifically in the urban environments of tropical countries. This study aims to estimate the prevalence of diarrhoeagenic Escherichia coli (DEC) and Salmonella spp. with zoonotic potential in urban slum environments. We trapped rats between April and June 2018 in Salvador, Brazil. We collected rectal swabs from Rattus spp., and cultured for E. coli and Salmonella spp., and screened E. coli isolates by polymerase chain reaction to identify pathotypes. E. coli were found in 70% of Rattus norvegicus and were found in four Rattus rattus. DEC were isolated in 31.3% of the 67 brown rats (R. norvegicus). The pathotypes detected more frequently were shiga toxin E. coli in 11.9%, followed by atypical enteropathogenic E. coli in 10.4% and enteroinvasive E. coli in 4.5%. From the five black rats (R. rattus), two presented DEC. Salmonella enterica was found in only one (1.4%) of 67 R. norvegicus. Our findings indicate that both R. norvegicus and R. rattus are host of DEC and, at lower prevalence, S. enterica, highlighting the importance of rodents as potential sources of pathogenic agents for humans.

Evidence of multiple intraspecific transmission routes for Leptospira acquisition in Norway rats (Rattus norvegicus).

Infectious diseases frequently have multiple potential routes of intraspecific transmission of pathogens within wildlife and other populations. For pathogens causing zoonotic diseases, knowing whether these transmission routes occur in the wild and their relative importance, is critical for understanding maintenance, improving control measures and ultimately preventing human disease. The Norway rat (Rattus norvegicus) is the primary reservoir of leptospirosis in the urban slums of Salvador, Brazil. There is biological evidence for potentially three different transmission routes of leptospire infection occurring in the rodent population. Using newly obtained prevalence data from rodents trapped at an urban slum field site, we present changes in cumulative risk of infection in relation to age-dependent transmission routes to infer which intra-specific transmission routes occur in the wild. We found that a significant proportion of animals leave the nest with infection and that the risk of infection increases throughout the lifetime of Norway rats. We did not observe a significant effect of sexual maturity on the risk of infection. In conclusion, our results suggest that vertical and environmental transmission of leptospirosis both occur in wild populations of Norway rats.

Factors affecting carriage and intensity of infection of Calodium hepaticum within Norway rats (Rattus norvegicus) from an urban slum environment in Salvador, Brazil.

Urban slum environments in the tropics are conducive to the proliferation and the spread of rodent-borne zoonotic pathogens to humans. Calodium hepaticum (Brancroft, 1893) is a zoonotic nematode known to infect a variety of mammalian hosts, including humans. Norway rats (Rattus norvegicus) are considered the most important mammalian host of C. hepaticum and are therefore a potentially useful species to inform estimates of the risk to humans living in urban slum environments. There is a lack of studies systematically evaluating the role of demographic and environmental factors that influence both carriage and intensity of infection of C. hepaticum in rodents from urban slum areas within tropical regions. Carriage and the intensity of infection of C. hepaticum were studied in 402 Norway rats over a 2-year period in an urban slum in Salvador, Brazil. Overall, prevalence in Norway rats was 83% (337/402). Independent risk factors for C. hepaticum carriage in R. norvegicus were age and valley of capture. Of those infected the proportion with gross liver involvement (i.e. >75% of the liver affected, a proxy for a high level intensity of infection), was low (8%, 26/337). Sixty soil samples were collected from ten locations to estimate levels of environmental contamination and provide information on the potential risk to humans of contracting C. hepaticum from the environment. Sixty percent (6/10) of the sites were contaminated with C. hepaticum. High carriage levels of C. hepaticum within Norway rats and sub-standard living conditions within slum areas may increase the risk to humans of exposure to the infective eggs of C. hepaticum. This study supports the need for further studies to assess whether humans are becoming infected within this community and whether C. hepaticum is posing a significant risk to human health.

Stress-host-parasite interactions: a vicious triangle?

The potential for synergy between host physiological condition and infection has recently been recognised. Here we review the evidence to support the idea that stress may play a key role in the interplay between host and parasite, integrating a three-way circular synergistic interaction between stressors, infections and host response: chronic stress can elicit responses that impoverish the host's physiological condition (including its immune function), which predisposes to infection, which results in more stress, and so on. We argue that this introduces additional explanatory power to previous ideas posited by the authors, by including stress as a third interacting factor that intervenes in the synergy between condition and infection. This in turn, may have important implications. In nature, evolutionary forces appear to select against stress-related disease or exacerbated parasite virulence. Nonetheless, under certain circumstances, parasites and (other) stressors interact generating a vicious spiral that may affect host fitness and survival. At high host densities, this becomes a mechanism of population regulation. Also, anthropogenic stressors may cause this mechanism to misfire, with significant implications for biological conservation and public health.

El potencial sinergismo entre la condición fisiológica del hospedador y la infección ha sido recientemente reconocido. Aquí revisamos la evidencia que da sustento a la noción que el estrés puede jugar un papel clave en la interacción entre el hospedador y el parásito, integrando una interacción circular sinérgica de tres vías que comprende al factor estresante, las infecciones y la respuesta del hospedador: el estrés crónico desencadena respuestas que empobrecen la condición fisiológica del hospedador (incluyendo su función inmune), lo cual predispone a infecciones, lo cual resulta en más estrés, y así sucesivamente. Sostenemos que esta noción introduce capacidad adicional para explicar las ideas previas propuestas por los autores, mediante la incorporación del estrés como un tercer factor interactuante que interviene en el sinergismo entre condición e infección. A su vez, esto puede tener importantes implicancias. En la naturaleza, fuerzas evolutivas parecen remover a enfermedades relacionadas al estrés o a la virulencia exacerbada. No obstante, bajo ciertas circunstancias, los parásitos y (otros) factores estresantes interactúan generando círculos viciosos que pueden afectar la sobrevida del hospedador. A altas densidades, este se vuelve un mecanismo de regulación poblacional. Asimismo, factores estresantes antropogénicos pueden causar que este mecanismo se dispare en falso, con implicancias significativas paraa la conservación de la biodiversidad y la salud pública.